Speaking in parliament, Labour's shadow youth minister Steve Reed asked whether the government would "widen the scope" of the annual independent evaluation of the National Citizen Service (NCS) in order to "make comparisons with other youth programmes with similar aims to NCS".

But Crouch rejected the idea, adding that the government is already supporting efforts to improve evaluation of wider youth work.

This includes funding for the Centre for Youth Impact, a social enterprise that aims to improve how the youth sector measures its effectiveness.

She added that £1m in funding has been made available to evaluate projects funded through the government's £40m Youth Investment Fund.

"The youth sector evidence base is not yet sufficiently developed to enable robust comparison between different programmes," she said.

"The Department for Digital, Culture, Media and Sport (DCMS) is supporting youth organisations to develop the sector evidence base such as through funding for the Centre for Youth Impact and the £1m made available to evaluate the Youth Investment Fund."

Reed also asked if the government would give other youth programmes access to details of the control group of non-NCS participants used in evaluation of the scheme.

But Crouch also rejected this and said no other youth services had approached the DCMS for this information.

The NCS, which launched in 2011, offers 15- to 17-year-olds a three- to four-week programme of activities over summer and autumn and is backed by £1.2bn in government funding up to 2020.

Evaluation of NCS programmes delivered in 2016 found that the scheme has a "very positive short-term impact" for young people taking part, in terms of their confidence, social skills and life satisfaction.

Tony Taylor, co-ordinator of campaign group In Defence of Youth Work, said it is "absurd" to scrutinise the National Citizen Service in isolation from the diversity of continuing youth provision.

"Contrary to the claim that there is no evidence base to inform a thorough-going evaluation of practice, a range of insightful research is available, the latest being the 2017 Anu Gretschel report on the impact of International Youth Work," Taylor said.

"However, this body of knowledge has been wilfully ignored. Its qualitative perspective is utterly at odds with the government's neoliberal obsession with measuring the immeasurable.

"Though we do not think the way forward lies in some sort of crude, comparative exercise. As of now we see a strong case for using the funding - some £400m - that could be saved from a reduction by a third in NCS's recruitment target up to 2020/21 to reinstate the nearly £390m cut in youth service spending since 2010.

"The urgent longer-term need is for an independent inquiry into the present state of youth work in its entirety, premised on a renewed understanding of youth work as a distinctive educational practice rooted in voluntary relationships with young people forged outside of formal institutions and agencies."